1. Field
The following description relates to a network protection switching technology that enables switching from an operation network to a protection network in response to an error in the operation network, and more particularly to a fast protection switching method in a Passive Optical Network.
2. Description of the Related Art
According to the 10-Gigabit-capable Passive Optical Networks (XG-PON) standard, an Optical Line Terminal (OLT) periodically transmits an upstream bandwidth map to an Optical Network Terminal (ONT) or an Optical Network Unit (ONU) in a Time Division Multiplexing (TDM)-based XG-PON system. The upstream bandwidth map is time map information necessary for each ONT to transmit data in upstream (that is, from the ONT to the OLT).
The OLT calculates a value of Round Trip Delay (RTD) from the OLT to the ONU with respect to each ONT through a ranging process, assumes that all the ONTs are away for the same distance (e.g., 20 km), and transmits delay information to each of the ONTs. The delay information refers to Equalization Delay (EqD).
An optical network protection switching structure includes a PON link entirely duplex structure and a PON link partially duplex structure, as illustrated in FIGS. 1 and 2. Referring to FIG. 1, the PON link entirely duplex structure is configured to have two independent PON links for an operation network and a protection network, which are used as an operation link and a protection link, respectively. The PON link entirely duplex structure needs to correspond to independent PON links in two ONT systems, the characteristic that requires high-cost configurations (a Medial Access Control (MAC) chip and an optical transceiver) and duplex management.
The PON link partially duplex structure has a different feeder line, so that an ONT does not have a device for a protection link. Thus, the PON link partially duplex structure may be established with lost costs compared to the PON link entirely duplex structure, but unable to perform protection switching for a distribution network.
The technical object of the optical network protection switching technique is to perform protection switching to restore a link without affecting a service provided to a user. That is, it needs to perform protection switching without affecting a service provided to a user.
In response to a loss of signal (LOS) alarm (which is described in G.987.3 regarding XG-PON), an OLT in an operation link is switched to a protection link. The switching time includes backup time for existing subscriber information and time for correcting a change in distance to an ONT.
An OLT in a protection link needs to re-calculate Round Trip Delay (RTD) for each ONT and transmit an EqD value to each ONT. According to the XG-PON standard, RTD for ONTs are calculated in sequence, so that it is hard to calculate EqD values for ONTs within a short time.
Among various conventional methods for correcting a change in distance to an ONT, there is a method of transmitting a broadcast Physical Layer Operations, Administration and Maintenance (PLOAM) message including differences in RTD between all the ONTs. This method utilizes the characteristic that a distance between an operation link and a protection link exists only in the feeder network in the PON link partially duplex structure and thus there is no difference in RTD between all the ONTs. The PLOAM message is a control message transmitted and received between an OLT and an ONT, which is described in G.987.3).
However, a distance from an OLT to each ONT may differ in the PON link entirely duplex structure, so it is impossible to apply the above-described method thereto. To avoid the distance problem, Korean Patent No. 10-0833504 (May 23, 2008) has been proposed, which makes a distance between each operation link and each protection link to be the same. However, it is difficult to make a distance between each operation link and each protection link to be the same, and the operation network and the protection network generally have different loop locations.
Therefore, there was a need for studying a technology that solves a technical problem of fast switching between an operation network and a protection network, by rapidly updating Equalization Delay (EqD) values when performing protection switching from an operation link (an operation network) to a protection link (a protection network) in a PON, even if the EqD values are different for Optical Network Terminals (ONTs) of varying distances.